Apparatus for measuring electricity



Dec. 1.192s. 1,564,046

H. COOCH APPARATUS FOR MEASURING ELECTRICITY Original Filed April 1 1925 Patented Dec. 1, 1925.

UNITED STATES PATENT OFFICE.

HERBERT COOCH, OF SOUTH FARNBOR-OUGH, ENGLAND, 'ASSIGNOR TO VICKERS LIM ITED, OF WESTMINSTER, ENGLAND, A BRITISH COMPANY.

APPARATUS FOR MEASURING ELECTRICITY.

Original application filed April 17, 1925, Serial No. 23,822. Divided and this application filed October Serial No- 63,822.

'1 '0 all whom it may concern Be it known that 1, Hammer Coooi-r, a subject of the King of Great Britain, residing at Royal Aircraft Establishment, South Farnborough, in the county of I-Iants, England, have invented certain new and useful Improvements in Apparatus for Measuring Electricity, of which the following is a specific'ation.

This invention relates to an improved apparatus for measuring electricity and particularly for measuring small alternating electric current and is a division of application Ser. No. 28822 filed April 17, 1925. The invention primarily is intended for use in measuring the strength of current or electrolnotive force induced in search coils or other conductors on a ship, airplane or other craft by a pulsating or alternating magnetic field determined by a leader cable or the like in the sea or on the ground in navigational "terns to indicate direction, or distance from or height above the cable in bad visibility. The invention however is applicable for measuring alternating current or E. M. F. in other systems.

In such navi ational systems, on the one hand, the E. F. induced in the search coils enclosing the pulsating or alternating magnetic field of any given strength can be increased proportionately by increasing the frequency of the magnetic field, but, on the other hand, whereas the amplitude of oscillation of a measuring device tuned to the same fretpiency as the applied E. M. F. is an indication of the strength of the E. M. E, the. power required to give a definite amplitude of oscillation of a measuring device is greater in a device tuned to a higher frequency than in a device responding to a lowfrequency.

According to the present invention, a small E. M. F. of higher frequency is measured by a device operating at a ower frequency after the higher frequency has been reduced to the lower frequency.

Further according to the present invention in a navigational apparatus wherein a search coil or the like conductor has alternating current of a relatively high frequency induced in it by a pulsating or alternating magnetic iield, a measuring device operating at a relatively low frequency is used-and a frequency reducer is interposed between the search coil and the measuring device to reduce the higher frequency to the lower frequency. V

The invention also comprises the association of a device operating at a lower frequency for measuring a small'E. M. vF. of higher frequency after reduction to the lower frequency with a device forreducing the higher frequency E. M. F. to the lower frequency E. M. F. consisting of a vibrating reed, tuning fork, clockwork or other periodic electric contact apparatus having a natural frequency equal to the sum of or the difference between said frequencies.

The invention further. comprises for the purpose of measuring the strength of an E. M. F. which has had its frequency reduced, the use of a measuring device, such as a mirror and a tangent scale, in which larger linear height or other indications are obtained per unit angular movement of the mirror at the zero end of the scale, and smaller linear height or other indications are obtained per unit angular movement of the mirror at the maximum end of the scale.

The invention further comprises navigational apparatus constituted by the combination with frequency reducing and measuring apparatus as aforesaid of two search coils or sets of search coils symmetrically disposed about a fore-and-aft vertical plane and inclined to the horizontal plane and to each other. In such an arrangement, two scales may be associated side by side or in other convenient arrangement to enable the relative strengths of the alternating current or E. M. F.s induced in the search coils and the relative phase or polarity of the currents or E. M. F.s in the search coils. to be observed.

The invention is not limited to any'particular frequencies of E. M. F. to be measured or any particular frequency applied to the measuring device, but in a navigational system of the nature referred to the fire quency of the E. M. F. in the search coils may be of the order of say 25 to 100 cycles per second, and the frequency applied to the measuring device may be' of the order of say 2 cycles per second.

In'car'rying out the invention, the frequency reducer may be connected in series between the search coil or other circuit car rying the E. M. F. to be measured and the measuring apparatus.

The frequency reducer mayv be of any suitable type such for example as clockwork, vibrating reed, tuning fork or any apparatus which will close and open an electric contact periodically at any predetermined sequence. Where the frequency reducer is of the type which closes thecircuit intermittently between the source of E. M. F. to be measured and the measuring device the fre quency of the contact on the reducer is tuned to be equal to thefrequencyofthe phenome non to be measured plus or minus the natural frequency ofthemeasuring device. The frequency of the current. supplied to the measuring device is thus equal to the difference between the frequency of the E. M. F. to be measured and. thefr'equency of the reducer contact.

Condensers may be connected on each or one-side of the frequency reducer in parallel with either the measuring device or the source of the E, M. F. to be measured or with both to assist by their discharge action the supply of EVM. F. to the measuring device. The capacity of the condensers may be such that the search coil and one condenser, and the measuring instrument and the other condenser from two electric oscillatory circuits which are-connected together intermittently by the frequency reducer.

The measuringdevice may be of any type of apparatus for indicating strength of current or E. 'M. F. provided it"has a suitably low natural period ofoscillation. It may be a centre zero direct current indicator which operates at=a predetermined current or E. M. F.

In applying the invention to a navigational system such as indicatedabove comprising two separate inclined search coils are arranged at anangle-t-o one another and acted on by-the pulsating or alternating! magnetic field from a sea or ground cable. By utilizing higher frequencies in the sea-or ground cable to givea higher induced E. M. F. inthe searoh coils, Q ieuveight of'thje search coils carried by the aircraft as well as the-currentinthe sea or ground cable can be reduced.

The application of the present invention enables the control 'relayand indicating means hitherto 1 employed to be replaced by a low frequency direct reading device giving indicationsof'the strength-of the E. M. F. induced in-the-searchcoils. In this case the indicating instrument, for example; may consist of; a pair of coils-moving in strong un -directional"magnetic fields similar to the well know methods adopted for voltnieters butpreferably wound; "on =no'n-fconduc farms t c reduce tlie danipingeffect. he measuring device can be tuned to a low natural period of oscillation so that an appreciable amplitude of swing results when very small alternating currents of the same frequency are applied. The amplitude of swing can be calibrated to indicate the E. M. F. applied.

The amplitude of swingof the measuring device may be indicated by suitable pointers attached to the moving coilsor other moving part, and moving over scales in the well known manner, or the amplitude of swing or angular movement of the coils or other moving part may be indicated by mounting a mirror on each moving coil and recording the angular movements by spots of light reflected from the mirrors on to a suitable scale.

The density of the magnetic field varies approximately inversely as the distance from the ground cable and it is an advantage tov have larger indications per unit M. F. induced in the search coils when. in a weak field or at some distance from the cable than when the search coils are in a stronger field near the ground cable.

In the method of recording the angular movement of the measuring coils by spots of light reflected from mirrors on to a scale, the larger linear movement of the spot of light on the scale per unit E. M. F. at the minimum reading end ofthe scale and the small linear movement perv unit E. M. F. at the maximum end of the scale may be obtained by arranging the scale in a plane which lies tangentially to the path of the moving coil. The natural frequency oi. swing of the moving coils is kept low so that only low powers are required to give an angular movement. The most suitable frequency is probably one where the speed of movement of the spot of light or pointer movement is fast'enough to give an optical illusion of'a quantitive streak and also enough to distinguish a 180 diil'erence in phase or direction of movement when two such indicationsare given side by side, By distinguishing this 180 difference in phase. indications are obtained of the relative polarity'of thesearch coils inthe alternating magneticfield,

In order to illustrate the invention more clearly its'operation will be described with reference .to the accompanying diagram matic drawings as applied to the measure ment of small alternating currents or M. F .s in navigational apparatuswhere search coils are carried by an aircraft and in which coils E. F..s are induced due to an alternati'ng or pulsating magnetic lield produced by currents in. a groundcablein the manner above indicated. H I

the constructionof apparatus shown. two searchceils 1,2, arecarri'ed bythe air craft being symmetrically lispesed about fore-and-aft 'vcrtical planev ABand inclined Ill) to the horizontal plane CD and to each other: by simultaneously recording E. M. F.s induced in the coils steering directions to follow a definite path and the height of the craft above the ground are indicated. Although the coils 1 and 2 are shown as interlinked this is not essential and they may be separated by any desired distance. When the E. lvl. FFs induced in the two search coils are unequal as indicated on the instrument the aircraft is not on its definite course and a steering direction is obtained to enable the aircraft to get on to the course, and when the E. M. F.s are equal the pilot has indication that he is on his correct course and his height above the ground is simultaneously indicated.

The numeral 3 indicates a frequency reducer consisting of four double pole reversing contacts 5, 6, two of which are connected to the coil 1 and two of which are connected to the coil 2. are coupled or otherwise arranged so that they are operated simultaneously by a vibrating reed, tuning fork, clockwork or other apparatus having a constant predetermined frequency. The current from the coil 1 passes through the frequency reducer 3 to a measuring instrument 9, and the current from the coil 2 passes throi'igh the frequency reducer 3 to a measuring instrument 10.

The two halves of the apparatus shown are similar and it will therefore suffice if one half is described in detail.

The frequency of the E. M. F. or current induced in the search coil 1 may be, for example, 83 periods per second. The frequency of the oscillating contacts 5 may be, for example, 35 or 31 periods per second.

The measuring instrument 9 consists of a coil 11 moving in a uni-directional magnetic lield in a similar manner to the well known arrangement adopted in voltmeters. The voltmeter movement used is of the usu'a-l centre zero type with a mirror 12 attached to the spindle in the position usually occupied by the pointer.

The movement of. the coil 11. operates against the action of springs in the well known manner of moving coil instruments, but in the present case the spring and the coil are adjusted to have a definite predetermined natural period of oscillation. This period of oscillation in the present CLSB is 2 periods per second or the difference between the frequency of the current induced in the search coil 1 and the frequency of the mov contacts of the frequency reducer 3.

A projector 13 causes a beam of light letto be reflected from the mirror 12 on to a? scale 15 shown so as to cause a spot or line of light to appear on the left hand side of the graduated scale 15 in a position depending on the angular position of the mirror.

The contacts 5, 6

The scale is arranged tangentially to the path of the moving coil and the projector is arranged so that the spot of light is at the top end of the scale when the coil is stationary or in Zero position.

It will be appreciated that as the moving coil oscillates about its stationary or zero position the spot of light oscillates up and down the scale, but as when the coil in its stationary or Zero position, the spot of light is at the top or one end of the scale, only one half of the oscillation of the spot of light is visible on the scale. The distance travelled by the spot of light down the scale indicates the amplitude of oscillation of the moving coil. The scale is set tangentially to the path of the moving coil as shown in the drawing so that a large linear movement per unit angular coil movement is obtained at the top end of the scale and a smaller linear movement per unit angular coil movement is obtained at the bottom end of the scale.

The scale is graduated as shown to indicate height above the ground. cable and as the E. M. F. induced in the coil is minimum at maximum height and maximum at mini mum height, the'greatest height recorded will be at the top end of the scale orin the position where there is minimum movement on the moving coil.

If the alternating EM. F. at 33 periods per second from the search coil 1 is passed through the double pole reversing contacts 5 of the frequency reducer 3 which oscillates at a frequency of 31 periodsper secend, the resultant E. M. F. applied to the measuring instrument 9 has an average value which oscillates at the rate of 2 periods per second. The moving coil of the measuring instrument 9 therefore receives electrical impulses at a rate equivalent to its own natural frequency of oscillation and the amplitude of oscillation is an indication of the strength of the E. M. F. induced in the search coil. It will also be appreciated that the average current applied to the measuring instrun'ient 9 is in one direction when the contacts of the fretpiency reducer reverse most of the negative portion of the alternating curve of the current received from the search coil, and the average current in the instrument is reversed when the frequency reducer reverses most of the positive portion of the alternating curve of the current received from the search coil.

lf now we examine the operation of the complete apparatus shown in the drawing it will be seen that the E. M. F. induced in the search coil 1 at a. frequency of 38 periods per second causes a spot of light to oscillate on the left hand side of the graduated scale at the rate of 2 periods per second, the amplitude of its oscillation being governed by the amount of E. M. F. induced in the search coil 1. Similarly the E. M. F. induced in the search coil 2 causes a second spot of light to oscillate on the right hand side'of the graduated scale at an amplitude governed by the amount of E. M. F. induced in the search coil 2. The two pairs of reversing contacts of the frequency reducer 3 operate simultaneously and the connections are arran ed so that the two spots of light are in synchronism when the two search coils are arranged so that at any instant the magnetic field passes through one coil from the under side and through the second coil from the top side. If for example the search coils are placed in a magnetic field which is horizontal and in the same plane as the paper of the drawing,

netic field enters from the top side of one coil and the underside of the second coil.

to the other, which are of advantage and of great value in navigational apparatus of this type.

What I claim and desire to secure by Letters Patent of the United States is 1. Means for the measurement of alternatng current phenomena, comprising in combination, a coil in which alternating current from a field may be induced, said coil being mounted for bodily movement in all directions so as to present varying areas to the field and thereby varying the E. M. F. of the current induced, a frequency reducer and a measuring device operating at thereduced frequency and responsive to the E. M. F. of the current induced.

2. Means for the measurement of alternating current phenomena, comprising in combination, a coil in which alternating current from a field may be induced, said coil being mounted for bodily movement in all directions so as to present varying areas to the field and thereby varying the E. M. F. of the current induced, a frequency reducer, a measuring device operating at the reduced frequency and responsive to the E. M. F. of the current induced, and a tangent scale for the measuring device, said tangent scale being so disposed that the length of indication obtained thereon per unit angular movement of said measuring device decreases progressively from the point on said scale corresponding with the Zero position of said measuring device to the point corre sponding With the maximum position of said measuring device.

HERBERT COOCH. 

